I'm more fluent as well when I talk to myself (100% fluent when singing). But, I still block or have repetitions at times even when alone. As for why we block when talking to others...not exactly sure. Nerves? My fluency also changes depending on who I'm talking to. With some people I'm quite fluent, with others I block on every other word. I've realized recently that often when I block I also stop breathing for that moment, so I'm thinking that has something to do with my blocks. So, maybe when we're talking with others we lose concentration on our breathing, thus leading to more frequent blocks? (Not saying that breathing issues cause blocks or stuttering, but maybe it's a factor?) Not sure. Just something I've been thinking about.